Wind noise is a serious problem in many hearing aids. Wind noise is caused by turbulent airflow over the microphone(s) in the hearing aid. Turbulence occurs when air flows around any obstacle, so it can never be entirely eliminated in a hearing aid placed on the head. Wind noise is often annoying while listening and can mask desired speech sounds. Severe wind noise can overload the A/D converter or the microphone pre-amplifier. When overload distortion occurs signal processing solutions will be ineffectual since the distortion occurs prior to the digital processing.
An airflow of 5 m/sec (11 miles/hour) will typically generate input-referred one-third-octave band sound-pressure levels of 75 to 100 dB SPL for a hearing aid mounted on a dummy head. The pressures are greatest for the wind at 0 deg (straight ahead), and lowest for the wind at 90 deg.
The wind noise signal has three basic characteristics. First, it is concentrated at low frequencies. The measurements of Wuttke, J. (1991, “Microphones and the wind”, J. Audio Eng. Soc, Vol. 40, pp 809-817) for a commercial recording microphone show a spectrum that is relatively flat below 100 Hz and with an approximately −12 dB/octave slope above 100 Hz. The measurements in Dillon, H., Roe, I., and Katch, R. (1999), “Wind noise in hearing aids: Mechanisms and measurements”, Nat. Acoustic Labs Australia, Report to Danavox, Phonak, Oticon, and Widex, 13 Jan. 1999 for various hearing aids show a wide variation in the wind-noise spectra, but the general behavior is a one-third-octave spectrum that is relatively flat below 300 Hz and inversely proportional to frequency above 300 Hz.
The spectrum of the wind noise also depends on the wind speed. Recordings of wind noise under a large number of wind conditions using a ReSound Canta 7 BTE attached to a DAT recorder are disclosed in Larsson, P., and Olsson, P. (2004), Master's thesis project, Lund Inst. Tech. The sampling rate was 48 kHz with 16-bit quantization. The recordings were made outdoors as they walked around the city of Lund, Sweden. The separate front and rear microphone signals were recorded simultaneously. Average power spectra for the front microphone for classifications of no wind, low wind speed (audible but not annoying), medium wind speed (troublesome), and high wind speed (uncomfortable or painful) are shown in FIG. 2. Each curve is the average of ten data files (approximately 3 minutes of data) for each wind-speed classification. The power spectra were computed by resampling the files at 16 kHz, applying a 1024-point Hamming window with fifty-percent segment overlap, and averaging the power spectra computed using 1024-point FFTs. The frequency resolution was 16.25 Hz.
The wind speed spectrum for no wind is limited at high frequencies by the noise floor of the hearing aid and recording apparatus, but some wind noise is apparent even for the no-wind condition where it could not readily be perceived. The low wind speed spectrum has a peak at about 32 Hz, and the peak frequency increases to about 100 Hz as the wind speed increases to high. All three curves for the wind present have a high-frequency slope of about −30 dB/decade, which lies between that of a 1-pole and a 2-pole low-pass filter.
The best procedure to reduce wind noise is to place a screen over the microphone ports to reduce the turbulence, and many effective windscreens have been developed for sound-recording microphones (Wuttke, J. (1991), “Microphones and the wind”, J. Audio Eng. Soc, Vol. 40, pp 809-817). But a screen may not be practical for a hearing aid given constraints on size or appearance, in which case an algorithmic solution is needed to reduce the wind-noise effects. Assuming that the microphone and pre-amplifier are not overloaded by the wind noise, signal processing can be effective in reducing the annoyance and masking effects. If, however, the major effect of the wind noise is overloading the microphone pre-amplifier, signal processing that occurs after the pre-amplifier will not reduce the noise problems.
Various schemes have been proposed for one-microphone noise suppression. Spectral subtraction (Boll, S. F. (1979), “Suppression of acoustic noise in speech using spectral subtraction”, IEEE Trans. Acoust. Speech and Sig. Proc., Vol. 27, pp 113-120), for example, estimates the noise power from the non-speech portions of the signal and subtracts the noise power from the total power in each frequency band. When wind noise is present, it will dominate the low-frequency power estimates, and spectral subtraction will therefore reduce the wind noise. Other techniques, such as reducing the gain in those frequency bands that have a low level of amplitude modulation, will also reduce wind noise. Even though these techniques are not designed specifically for wind-noise reduction, they will reduce the wind noise to some degree.
EP 1 519 626 discloses a system and method for detection and suppression of wind-noise in a hearing aid wherein a converted acoustic signal is processed in a number of frequency bands, a low-frequency band of which is selected as a so-called master band. The signal level of the master band is determined and compared to an absolute threshold value. The signal levels in the other frequency bands are also determined and compared to individual threshold values in each respective band. The signal level in each band is attenuated provided that the signal level in the master band is above the threshold value and the signal level in the band in question is also above its threshold value.
Thus, in EP 1 519 626, a signal level comparison is performed for each frequency band and based on the comparison attenuations in each respective band are performed, however threshold comparison and attenuation in every band is a computationally costly way of detecting and suppressing wind noise.
Further, the threshold comparison may lead to undesirable attenuation in listening situations with a low frequency signal that the hearing-aid user actually desires to hear, for example listening at an outdoor concert to music. Music typically includes low frequency sounds. In such a situation the method disclosed in EP 1 519 626 may undesirably reduce the low frequency gain in response to music of low frequency.